The present invention relates to an apparatus for processing an X-ray image, and more particularly to what is called an apparatus for digital subtraction angiography (DSA), in which video signals representing an X-ray fluoroscopic image obtained from an X-ray television system are digitalized and subjected to subtraction processing so as to obtain a vascular image.
There are some cases where X-ray images, such as contrast images obtained from an apparatus for DSA or vascular images obtained by subjecting the above-mentioned contrast images to contrast enhancement, are used as orientation images in surgical operations, etc. These contrast images and vascular images are characterized by a very large dynamic range, including the images of soft tissues where X-ray absorption is low and the images of skeletons where X-ray absorption is high. In order to ascertain vascularity on the surfaces of skeletons, etc., surgeons make an observation of low-density vascular images set off against the contrast images.
Vascular images are formed by contrast media, the X-ray absorption of which is lower than that of skeletons. Consequently the density of the vascular images is lower than that of the skeletal images, and vascularity is not easily ascertained.